Environmental regulations are requiring that the aromatics and sulfur content of diesel fuels be reduced. Reduction of the aromatics and sulfur content will result in less particulate and sulfur dioxide emissions from the burning of diesel fuels. Unfortunately, a hydrotreating catalyst that is optimized for hydrodesulfurization will not be optimized for aromatics saturation and vice versa. Applicant has developed a "stacked" or multiple bed hydrotreating system comprising a Ni-W/alumina catalyst "stacked" on top of a Co and/or Ni-Mo/alumina catalyst which offers both cost and activity advantages over the individual catalysts for combined hydrodesulfurization and aromatics saturation.
U.S. Pat. No. 3,392,112 discloses a two-stage hydrotreating process for sulfur-containing petroleum fractions wherein the first stage contains a sulfur-resistant catalyst such as nickel-tungsten supported on alumina and the second stage catalyst is reduced nickel composited with a diatomaceous earth such as kieselguhr.
U.S. Pat. No. 3,766,058 discloses a two-stage process for hydrodesulfurizing high-sulfur vacuum residues. In the first stage some of the sulfur is removed and some hydrogenation of feed occurs, preferably over a cobalt-molybdenum catalyst supported on a composite of ZnO and Al.sub.2 O.sub.3. In the second stage the effluent is treated under conditions to provide hydrocracking and desulfurization of asphaltenes and large resin molecules contained in the feed, preferably over molybdenum supported on alumina or silica, wherein the second catalyst has a greater average pore diameter than the first catalyst.
U.S. Pat. No. 3,876,530 teaches a multi-state catalytic hydrodesulfurization and hydrodemetallization of residual petroleum oil in which the initial stage catalyst has a relatively low proportion of hydrogenation metals and in which the final stage catalyst has a relatively high proportion of hydrogenation metals.
U.S. Pat. No. 4,016,067 discloses a dual bed hydrotreating process wherein in the first bed the catalytic metals are supported on delta or theta phase alumina and wherein both catalysts have particular requirements of pore distribution.
U.S. Pat. No. 4,016,069 discloses a two-stage process for hydrodesulfurizing metal- and sulfur-containing asphaltenic heavy oils with an interstage flashing step and with partial feed oil bypass around the first stage.
U.S. Pat. No. 4,016,070 also discloses a two-stage process with an interstage flashing step.
U.S. Pat. No. 4,012,330 teaches a two-bed hydrotreating process with additional hydrogen injection between the beds.
U.S. Pat. No. 4,048,060 discloses a two-stage hydrodesulfurization and hydrodemetallization process utilizing a different catalyst in each stage, wherein the second stage catalyst has a larger pore size than the first catalyst and a specific pore size distribution.
U.S. Pat. No. 4,166,026 teaches a two-step process wherein a heavy hydrocarbon oil containing large amounts of asphaltenes and heavy metals is hydrodemetallized and selectively cracked in the first step over a catalyst which contains one or more catalytic metals supported on a carrier composed mainly of magnesium silicate. The effluent from the first step, with or without separation of hydrogen-rich gas, is contacted with hydrogen in the presence of a catalyst containing one or more catalytic metals supported on a carrier preferably alumina or silica-alumina having a particular pore volume and pore size distribution. This two-step method is claimed to be more efficient than a conventional process wherein a residual oil is directly hydrosulfurized in a one-step treatment.
U.S. Pat. No. 4,392,945 discloses a two-stage hydrorefining process for treating heavy oils containing certain types of organic sulfur compounds by utilizing a specific sequence of catalysts with interstage removal of H.sub.2 S and NH.sub.3. A nickel-containing conventional hydrorefining catalyst is present in the first stage. A cobalt-containing conventional hydrorefining catalyst is present in the second stage.
U.S. Pat. No. 4,406,779 teaches a two-bed reactor for hydrodenitrification. The catalyst in the first bed can comprise, for example, phosphorus-promoted nickel and molybdenum on an alumina support and the catalyst for the second bed can comprise, for example, phosphorus-promoted nickel and molybdenum on a silica-containing support.
U.S. Pat. No. 4,421,633 teaches a multi-catalyst bed reactor containing a first bed large-pore catalyst having majority of its pores much larger than 100 .ANG. in diameter and a second bed of small-pore catalyst having a pore size distribution which is characterized by having substantially all pore less than 80 .ANG. in diameter.
U.S. Pat. No. 4,431,526 teaches a multi-catalyst bed system in which the first catalyst has an average pore diameter at least about 30 .ANG. larger than the second catalyst. Both catalysts have pore size distributions wherein at least about 90% of the pore volume is in pores from about 100 to 300 .ANG..
U.S. Pat. No. 4,447,314 teaches a multi-bed catalyst system in which the first catalyst has at least 60% of its pore volume in pores having diameters of about 100 to 200 .ANG. and a second catalyst having a quadralobe shape in at least 50% of its pore volume in pores having diameters of 30 to 100 .ANG..
U.S. Pat. Nos. 4,534,852 and 4,776,945 disclose that Ni/Mo/P and Co/Mo catalysts in a stacked bed arrangement provide significant advantages when hydrotreating certain types of coke-forming oils.